Carburetion issue, compression issue, or what?

[Jlmatt]

When in gear and the throttle 3/4 open, I can cruise just under hull speed in calm water. But when I open the throttle all the way I don't hear any change in rpm. Could that be caused by fuel starvation? I'm wondering because I removed the copper tubing between the pump and the carburetor and replaced it with a hose. The bend in that hose is quite sharp. I wonder if that might restrict the flow of fuel.
There is no way for me to compare because I never ran the engine under load with the original copper tubing.
Would a restriction in the fuel line cause that symptom?

[jhwelch]

You might want to redo your new fuel hose so there is no kink, but I don't think this is your problem -- it there was fuel starvation your engine would start to sputter, stumble, lose power, and shut down, a process that takes a couple of seconds.

I'm a but surprised that you are at 3/4 throttle. This seems much too high to me, but perhaps the control you are using only moves the throttle at the carb end a smaller amount.

It sounds like your issue might be slipping clutch plates, in which case (assuming good access) it is easy to remove the plate on top of your reversing gear and turn the adjusting collar one notch tighter. You can find instructions for this process elsewhere in this forum.

-Jonathan

[Mark S]

Can you tell us what size boat you're driving and what RPM produces what speed? While it seems logical that increasing throttle should increase RPM, for most of us it doesn't work that way once we've reached a speed of 6 knots or so, and increasing throttle just means burning more gasoline. I don't think your clutches are slipping because, if they were, increasing throttle at some point would cause the engine to race.

Mark

[Dave Neptune]

Normal

Jlmatt, what you describe is pretty normal for any direct drive A-4. You get the max power you can use because of the prop at around 3/4 throttle. If you advance the throttle more you will gain little af anything at all and the carb noise will change a bit. I'm using the Indigo prop for the additional RPM and power it will absorb" from the engine. I cruise at around 1/2 throttle at 2000 RPM's and get my max RPM's at around 3/4 throttle~~very normal.

RE a kink in the line yes it could restrict a bit but I don't think you are having that problem as you can get to 3/4 throttle, however I would try to get the line relaxed a bit if it is kinked. All that needs to flow is about a gallon an hour~not much.

Dave Neptune

[Jlmatt]

Thanks for your replies.
The boat is a Pearson Triton. The knot meter reads around 5 knots. I'll check with the gps next time. I don't have a tach so when I say there is no increase in rpm, I mean there is no difference in the sound of the engine when I open the throttle all the way.
If I did that in neutral it would scream.

Jean-Luc

[Carl-T705]

Are you quite certain that when you move the throttle lever to full you are actually opening the throttle blade in the carb? You may just be flexing the entire cable housing.... just a thought. Does the engine increase RPM when going to full in neutral? No to your question of fuel starvation at full throttle , the engine would miss and not maintain a steady RPM. PS I also own a Triton, #705

[msauntry]

Throttle position is kind of relative. I don't really have set positions on mine, but I know there is a point where more lever doesn't mean more RPM's. With an updraft carb, it can only take what it can take. Sounds normal and I wouldn't worry about it. 5 knots is a little slow, but I used to have a Triton and it was turning a 13x8 two blade which is too much prop for the boat. Any idea what prop you have?

[sastanley]

JLMatt, Dave Neptune spelled it out pretty good. In neutral, there is no load, the engine will scream to redline and beyond..in gear, it will only spin as fast as the prop (& hull characteristics for your particular boat) will let it..more throttle just wastes fuel.

I can tell when my prop & bottom are dirty based on my throttle position too...When everything is clean, the boat coasts along at barely above idle, but as the season wears on, I am applying more and more throttle to achieve the same RPMs..by the end of the season (if I haven't been diligent about cleaning the prop & hull), I can no longer get to hull speed, nor to my normal max RPM of 2,200...right now, I can only achieve about 1,900, and that requires more throttle than it did in the summer due to the dirty prop & hull.

[Dave Neptune]

Side Bar

Shawn, now that she's dirty what are your vac readings under load?

Dave Neptune

[sastanley]

Dave..I dunno, but I'll get you some numbers the next time I run her.

[Jlmatt]

I think I understand better now. I'm so used to a car engine and the way it reacts when you step on the accelerator. An inboard engine working with a propeller is a completely different thing.
Carl, my Triton is #259. And it reacts the same way when I push the arm directly at the carburetor so it's not a cable housing problem.
Msauntry, I don't know what my prop is. It's probably the one that came with the boat in 1961. Pardon my ignorance but when you say 13X8 I don't know what those numbers refer to. Mine has 2 blades and 13'' diameter sounds right.

Jean-Luc

[ndutton]

A quick primer on props

For those unfamiliar, prop dimensions are described as follows:

12 x 7 x 1 RH
you should find these numbers stamped on the prop hub.

The first number is the diameter in inches. When selecting a diameter be sure to allow a minimum of 1" between the tip of the prop and the hull at its closest point (for those of us with a strut mounted bearing). Any closer and you risk hull erosion.

The second number is the pitch. It represents the theoretical distance the prop would move forward with a single revolution at 100% efficiency. Think of it turning through a solid medium. Pitch can also be considered bite.

The third number is shaft diameter.

The letters represent right hand or left hand rotation. Viewed from the stern, a right hand prop turns clockwise to move the boat forward.

There are other descriptive tag-ons such as number of blades and blade shape. Blade shape often is encompassed by the manufacturer's product name such as sailor or super cup.

Diameter and pitch work in harmony to apply load to the engine. Selecting a prop is an inexact science, pretty much trial and refinement.

[positron]

My A4 has been acting the same- I can get it up to about 3/4 throttle and hit a max of 4.1 knots. Further increasing throttle has no discernible effect on either engine speed or boat speed. Earlier this spring I could hit the hull speed of around 6.0 with no problems, with a corresponding increase in engine speed. I rebuilt my fuel pump and carb this summer and the engine sat unused for 6-7 weeks. When I fired it up again, there is a new clicking sound which I attribute to a sticking valve. My theory is that a sticking intake valve is causing ~25% reduced power, and also messing up fuel/air intake for the other cylinders above 3/4 throttle. You will doubtless be hearing from me soon- the valves will be my first winter project when I haul the boat next week.

[Dave Neptune]

Oil 'em

Positron, before you lay up run her with some top oil in the fuel MMO or 2-stroke at around a 100:1 for an hour or so. This may just free up the valve if it is indeed sticking. Could just be a dirty bottom and prop and a little oil in the fuel before putting to rest will help with lubing what just spraying in can't accomplish. Everything is moving and getting exposed to the oil in the fuel when the engine is "running"~ a good thing.

Dave Neptune

[smosher]

sure sounds like a dirty prop and or bottom.

Steve